1. Field of the Invention
The present invention relates to transflective liquid crystal displays, and more particularly to single cell gap type transflective liquid crystal displays.
2. Description of the Prior Art
Liquid crystal display (LCD) types are divided into three kinds: a transmissive LCD, a reflective LCD, and a transflective LCD. However, the transmissive LCD is a non-effective light converter that merely transmits about 3% to 8% of light from the backlight. Therefore, the transmissive LCD requires a backlight device having high brightness, leading to high power consumption. The reflective LCD uses ambient light for imaging, thus saving power consumption. However, the reflective LCD can be used during the day or in environments where external light exists, but not during the night or under poor ambient lighting.
Therefore, the transflective LCD has been introduced. In general, two main approaches of transflective LCD have been developed: single cell gap (FIG. 1a) and double cell gap (FIG. 1b). FIG. 1a is a cross-section of a conventional transflective liquid crystal display using a single cell gap. The transflective LCD includes upper and lower substrates 200 and 100 opposing each other, and a liquid crystal layer 300 interposed between the upper and lower substrates. A common electrode 220 is formed below the upper substrate 200, a transmissive electrode 140 is formed in the transmission region T, and a reflective electrode 120 is formed in the reflection region R. The cell gap (d) in T and R regions is the same.
Light efficiency is proportional to the total retardation change experienced by the incident light traveling in the liquid crystal layer of the device. The total retardation change is a product of 1) birefringence change, Δn, ‘seen’ by the incident light as a result of the reorientation of the liquid crystal molecules upon an applied voltage and 2) total path length traveled by the incident light in the liquid crystal layer.
Since the light passes the LC layer 300 twice in the R region, but only once in the T region, the reflected light R experiences a total retardation change of (Δn)×(2d), twice that of T which is (Δn)×d. FIG. 1b shows that R reaches 100% brightness at 2.75V whereas T only reaches 50% at the same voltage.
In order to achieve high light efficiency for both R and T modes, the double cell gap approach is often used such that the cell gap in the R region is reduced to d/2, so that the total length traveled by light in the LC layer 300 for T and R regions is the same (FIG. 2a). The total retardation change in the R region, which is (Δn)×2×(d/2), is thus equal to that in the T region ((Δn)×d). Thus, both R and T can have equally high efficiency of 100% as shown in FIG. 2b. This approach, however, leads to much more complicated structure. The fabrication process needs to maintain good control over the difference between the two cell gaps, which depends on the control of the extra layer (usually an organic layer). Moreover, this difference in cell gap between R and T regions also leads to different response times between R and T modes.
U.S. Patent Publication US2003/0202139A1 discloses a transflective liquid crystal display that requires only a single cell gap. The disclosure in this publication is rather unclear. The abstract of this publication states that instead of reducing the cell gap of the reflective region, it reduces the birefringence change Δn of reflective pixels so that the total retardation change Δnd of the reflective region is equal to that of the transmissive pixels. This is realized by a partial switching of the pixels of approximately 45 degrees which occurs in the reflective pixel region of the single cell gap by applying fringing fields, generated by a discontinuous electrode, to the molecules in the reflective pixel region of the cell gap. It appears that the entire reflective region is provided with a discontinuous electrode in the structure disclosed in this publication. It is unclear how commercially viable would be the disclosed structure.